Re: [PATCH 4/4] memcg: get rid of percpu_charge_mutex lock

From: Michal Hocko
Date: Mon Aug 08 2011 - 17:47:15 EST

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On Mon 08-08-11 20:47:38, Johannes Weiner wrote:
[...]
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2071,7 +2071,6 @@ struct memcg_stock_pcp {
> > #define FLUSHING_CACHED_CHARGE (0)
> > };
> > static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> > -static DEFINE_MUTEX(percpu_charge_mutex);
> >
> > /*
> > * Try to consume stocked charge on this cpu. If success, one page is consumed
> > @@ -2178,7 +2177,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
> >
> > for_each_online_cpu(cpu) {
> > struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> > - if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> > + if (mem_cgroup_same_or_subtree(root_mem, stock->cached) &&
> > + test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> > flush_work(&stock->work);
> > }
> > out:
>
> This hunk triggers a crash for me, as the draining is already done and
> stock->cached reset to NULL when dereferenced here. Oops is attached.

Thanks for catching this. We are racing synchronous drain from
force_empty and async drain from reclaim, I guess. Sync. checked
whether it should wait for the work and the cache got drained and
set to NULL.
First of all we must not dereference the cached mem without
FLUSHING_CACHED_CHARGE bit test. We have to be sure that there is some
draining on that cache. stock->cached is set to NULL before we clear the
bit (I guess we need to add a barrier into drain_local_stock). So we should
see mem either as NULL or still valid (I have to think some more about
"still valid" part - maybe we will need rcu_read_lock).

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f4ec4e7..626c916 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2197,8 +2197,10 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)

for_each_online_cpu(cpu) {
struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
- if (mem_cgroup_same_or_subtree(root_mem, stock->cached) &&
- test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+ struct mem_cgroup *mem = stock->cached;
+ if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags) &&
+ mem && mem_cgroup_same_or_subtree(root_mem, mem)
+ )
flush_work(&stock->work);
}
out:

>
> We have this loop in drain_all_stock():
>
> for_each_online_cpu(cpu) {
> struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> struct mem_cgroup *mem;
>
> mem = stock->cached;
> if (!mem || !stock->nr_pages)
> continue;
> if (!mem_cgroup_same_or_subtree(root_mem, mem))
> continue;
> if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> if (cpu == curcpu)
> drain_local_stock(&stock->work);
> else
> schedule_work_on(cpu, &stock->work);
> }
> }
>
> The only thing that stabilizes stock->cached is the knowledge that
> there are still pages accounted to the memcg.

Yes you are right we have to set FLUSHING_CACHED_CHARGE before nr_pages
check (and do the appropriate cleanup on the continue paths). This looks
quite ugly, though.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f4ec4e7..eca46141 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2179,17 +2179,23 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
struct mem_cgroup *mem;

+ /* Try to lock the cache */
+ if(test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+ continue;
+
mem = stock->cached;
if (!mem || !stock->nr_pages)
- continue;
+ goto unlock_cache;
if (!mem_cgroup_same_or_subtree(root_mem, mem))
- continue;
- if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
- if (cpu == curcpu)
- drain_local_stock(&stock->work);
- else
- schedule_work_on(cpu, &stock->work);
- }
+ goto unlock_cache;
+
+ if (cpu == curcpu)
+ drain_local_stock(&stock->work);
+ else
+ schedule_work_on(cpu, &stock->work);
+ continue;
+unlock_cache:
+ clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);

^^^^^
need a barrier?
}

if (!sync)

> Without the mutex serializing this code, can't there be a concurrent
> execution that leads to stock->cached being drained, becoming empty
> and freed by someone else between the stock->nr_pages check and the
> ancestor check, resulting in use after free?
>
> What makes stock->cached safe to dereference?

We are using FLUSHING_CACHED_CHARGE as a lock for local draining. I
guess it should be sufficient.

mutex which was used previously caused that async draining was exclusive
so a root_mem that has potentially many relevant caches has to back off
because other mem wants to clear the cache on the same CPU.

I will think about this tomorrow (with fresh eyes). I think we should be
able to be without mutex.

Anyway thanks for the really good report!

>
> [ 2313.442944] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> [ 2313.443935] IP: [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
> [ 2313.443935] PGD 4ae7a067 PUD 4adc4067 PMD 0
> [ 2313.443935] Oops: 0000 [#1] PREEMPT SMP
> [ 2313.443935] CPU 0
> [ 2313.443935] Pid: 19677, comm: rmdir Tainted: G W 3.0.0-mm1-00188-gf38d32b #35 ECS MCP61M-M3/MCP61M-M3
> [ 2313.443935] RIP: 0010:[<ffffffff81083b70>] [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
> [ 2313.443935] RSP: 0018:ffff880077b09c88 EFLAGS: 00010202
> [ 2313.443935] RAX: ffff8800781bb310 RBX: 0000000000000000 RCX: 000000000000003e
> [ 2313.443935] RDX: 0000000000000000 RSI: ffff8800779f7c00 RDI: 0000000000000000
> [ 2313.443935] RBP: ffff880077b09c98 R08: ffffffff818a4e88 R09: 0000000000000000
> [ 2313.443935] R10: 0000000000000000 R11: dead000000100100 R12: ffff8800779f7c00
> [ 2313.443935] R13: ffff8800779f7c00 R14: 0000000000000000 R15: ffff88007bc0eb80
> [ 2313.443935] FS: 00007f5d689ec720(0000) GS:ffff88007bc00000(0000) knlGS:0000000000000000
> [ 2313.443935] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [ 2313.443935] CR2: 0000000000000018 CR3: 000000004ad57000 CR4: 00000000000006f0
> [ 2313.443935] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 2313.443935] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [ 2313.443935] Process rmdir (pid: 19677, threadinfo ffff880077b08000, task ffff8800781bb310)
> [ 2313.443935] Stack:
> [ 2313.443935] ffffffff818a4e88 000000000000eb80 ffff880077b09ca8 ffffffff810feba3
> [ 2313.443935] ffff880077b09d08 ffffffff810feccf ffff880077b09cf8 0000000000000001
> [ 2313.443935] ffff88007bd0eb80 0000000000000001 ffff880077af2000 0000000000000000
> [ 2313.443935] Call Trace:
> [ 2313.443935] [<ffffffff810feba3>] mem_cgroup_same_or_subtree+0x33/0x40
> [ 2313.443935] [<ffffffff810feccf>] drain_all_stock+0x11f/0x170
> [ 2313.443935] [<ffffffff81103211>] mem_cgroup_force_empty+0x231/0x6d0
> [ 2313.443935] [<ffffffff81111872>] ? path_put+0x22/0x30
> [ 2313.443935] [<ffffffff8111c925>] ? __d_lookup+0xb5/0x170
> [ 2313.443935] [<ffffffff811036c4>] mem_cgroup_pre_destroy+0x14/0x20
> [ 2313.443935] [<ffffffff81080559>] cgroup_rmdir+0xb9/0x500
> [ 2313.443935] [<ffffffff81063990>] ? abort_exclusive_wait+0xb0/0xb0
> [ 2313.443935] [<ffffffff81114d26>] vfs_rmdir+0x86/0xe0
> [ 2313.443935] [<ffffffff811233d3>] ? mnt_want_write+0x43/0x80
> [ 2313.443935] [<ffffffff81114e7b>] do_rmdir+0xfb/0x110
> [ 2313.443935] [<ffffffff81114ea6>] sys_rmdir+0x16/0x20
> [ 2313.443935] [<ffffffff8154d76b>] system_call_fastpath+0x16/0x1b
> [ 2313.443935] Code: b7 42 0a 5d c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec 10 48 89 5d f0 4c 89 65 f8 66 66 66 66 90 48 89 fb 49 89 f4 e8 10 85 00 00
> [ 2313.443935] 8b 43 18 49 8b 54 24 18 48 85 d2 74 05 48 85 c0 75 15 31 db
>
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--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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Next message: Per Forlin: "[PATCH v3] usb: gadget: storage_common: make FSG_NUM_BUFFERS variable size"
Previous message: Tony Luck: "Re: Missing put_cpu() in arch/ia64/sn/kernel/sn2/sn_hwperf.c?"
In reply to: Johannes Weiner: "Re: [PATCH 4/4] memcg: get rid of percpu_charge_mutex lock"
Next in thread: Johannes Weiner: "Re: [PATCH 4/4] memcg: get rid of percpu_charge_mutex lock"
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